A persistent problem in the use of thermally insulated ducts, that is, ducts formed of relatively thick but somewhat fragile fiber insulation, is the difficulty in attaching fittings to the side panels of the duct. In air distribution systems, for example a main, usually rectangular duct has attached to it, at various places along its length fittings providing branch ducts, direction change elbows, etc. A conventional method of attaching fittings to sheet metal ducts is illustrated in U.S. Pat. Nos. 2,874,980 and 2,963,783. Tabs on the end margin of the fitting are bent back sharply against the interior surface of the sheet metal duct adjacent the duct opening into which the fitting is inserted, the tabs, after bending, serving to lock the fitting to the duct wall. This form of attachment cannot be satisfactorily utilized when the duct is formed of mineral fiber insulation because the somewhat fragile duct wall does not present a sufficiently hard and rigid surface at the fitting margin to permit the fitting tabs to be bent over the duct wall. Tape with an adhesive face has heretofore been used to secure fittings to insulation board panels of ducts, however, this type of joint causes difficulty when humidifiers are installed on the heating plant or when subjected to salt air or other somewhat corrosive atmosphere. The tape, after a time, loses its adhesion, the fitting loosens and leaks develop.
The concept of the present invention provides an economically formed and conveniently applied means for securely attaching a fitting to a side panel of an insulated duct without using adhesive backed tape.